What are van der Waals’ forces?
van der Waals’ forces are weak attractive or repulsive forces between molecules or atoms that are caused by a temporary uneven distribution of electron clouds.
Noble gases such as neon and argon exist as single atoms and do not form covalent bonds because they have full outer-shells. However, the fact that noble gases can be liquefied at very low temperatures, means there must be very weak forces of attraction between their atoms. These weak forces of attraction keep the atoms together in
the liquid state.
Bromine is a non-polar molecule because it is made up of 2 similar atoms that have the same electronegativity. Bromine is a liquid at room temperature, because of the weak forces of attraction that are keeping the bromine molecules together at room temperature.
These very weak forces of attraction are called van der Waals’ forces and they exist between all atoms or molecules.
Formation of van der Waals’ forces
The electron charge clouds in a non-polar molecule or atom are in constant motion. Sometimes there is more of the charge cloud on one end of the molecule than the other, making that end more negative charge than the other and thereby creating a temporary dipole.
This temporary dipole can then induce a dipole on neighbouring molecules as the δ+ end of the dipole in one molecule attracting the δ– end of the dipole in a neighbouring molecule. These dipoles are temporary because the electrons clouds are constantly moving.
van der Waals’ forces are sometimes called temporary dipole–induced dipole forces.
van der Waals’ forces increase with:
- increasing number of electrons in the molecule
- increasing the number of contact points where the molecules come close together.
Characteristics of van der Waals forces
Characteristics of van der Waals forces include:
- They are weaker than normal covalent and ionic bonds.
- They have no directional characteristic.
- They are short-range forces.
- Van der Waals’ attraction is greater if the molecules are closer.
Effects of van der Waals’ forces on enthalpy change of vaporisation and boiling points of the noble gases
Sizes of the van der Waals’ forces can be used to explain the trends in the enthalpy change of vaporisation and boiling points of the noble gases. The enthalpy change of vaporisation is the amount of energy required to convert a mole of liquid into a mole of gas.
In noble gases, both the enthalpy change of vaporisation and the boiling points increase as the number of electrons increases. van der Waals’ forces between the atoms are increases with an increasing number of electrons and so more energy is needed to change the liquid into vapour, raising the boiling point.
The effect increasing the number of contact points of molecules on van der Waal’s forces
The effect of increasing the number of contact points is seen by comparing the boiling points of pentane (boiling point 36°C) and 2,2-dimethylpropane (boiling point 10°C). These compounds have the same numbers of electrons in their molecules.
The molecules of pentane can line up beside each other so there is a large number of contact points increasing the van der Waals’ forces. So the boiling point is higher.
The molecules of 2,2-dimethylpropane are more compact and have less contact points. The van der Waals’ forces are therefore relatively lower and so the boiling point is lower.